1. Field of the Invention
The present invention relates to a (co)polycarbonate composition mainly comprising a (co)polycarbonate and a process for preparation thereof. More particularly, the present invention relates to a discoloration-free, chlorine-free (co)polycarbonate composition mainly comprising a heat-resistant, linear (co)polycarbonate which is free from any branched structure which might be formed by a side reaction during polycondensation, and a process for preparation thereof, and another (co)polycarbonate composition mainly comprising a heat-resistant, branched (co)polycarbonate which does not contain chlorine causative of discoloration and is suitable for the production of a hollow molding, and an efficient process for preparation thereof.
2. Description of the Related Art
A high-molecular-weight polycarbonate is a general-purpose engineering thermoplastic which is useful in various fields, particularly as an injection molding material or a sheet material substituting for a window pane.
A polycarbonate is prepared by the interfacial polycondensation process (i.e., the phosgene process) which comprises adding a solvent such as methylene chloride to an aqueous solution or suspension of a sodium salt of a dihydric phenol and blowing phosgene into the obtained system to conduct a reaction, or by the transesterification process which comprises heat-melting a dihydric phenol and a carbonic diester such as diphenyl carbonate and conducting the polycondensation thereof through transesterification at a high temperature and under a reduced pressure.
Among the above processes, the interfacial polycondensation process is generally widely utilized. However, according to this process, not only the use of highly toxic phosgene is necessitated, but also chlorine remains in a reaction product mainly comprising a produced polycarbonate. When chlorine remains in the reaction product mainly comprising a polycarbonate, the reaction product discolors during the molding thereof at high temperatures. Accordingly, the obtained reaction product must be washed to lower the residual chlorine concentration.
On the other hand, the transesterification process has advantages in that the use of highly toxic phosgene is not necessitated and that the provision of a step of removing residual chlorine is unnecessary. The transesterification process is generally effected in the presence of a catalyst and at a high temperature under reduced pressure. As catalysts useful for the preparation of a polycarbonate by the transesterification process, alkali metal compounds and alkaline earth metal compounds are well known. However, these compounds are known to cause side reactions leading to a branched structure through the Kolbe-Schmitt reaction or the formation of an isoalkenylphenol [see L. Bottenbruch, Encyclopedia of Polymer Science and Technology, 10, 722 (1969)]. Accordingly, when the transesterification process is conducted by the use of the above catalyst for the purpose of preparing a high-molecular-weight polycarbonate, a branched structure is formed by the side reaction, so that the product obtained under certain reaction conditions is partially insoluble in a solvent such as methylene chloride, and/or, is significantly discolored.
Meanwhile, since polycarbonates of the prior art exhibited Newtonian fluid characteristics in a molten state, the molding thereof into a hollow article was difficult. It has been known that the melt viscosity of a polycarbonate under a high shear rate decreases and that under a low shear rate increases by imparting a branched structure to the polycarbonate [see D. FREITAG et al., Encyclopedia of Polymer Science and Engineering, 11, 660 (1988)]. Therefore, the moldability of a polycarbonate into a hollow article can be improved by utilizing this fact. Thus, it is a practice to impart a branched structure to a polycarbonate by using a polyfunctional organic compound having at least three functional groups as a branching agent in the preparation of a polycarbonate.
For example, in U.S. Pat. Nos. 5,104,964 (assignee: IDEMITSU PETROCHEM K.K.; date of patent: Apr. 14, 1992) and 5,283,314 (assignee: IDEMITSU PETROCHEM K.K.; date of patent: Feb. 1, 1994), a branched polycarbonate is prepared by using a polyfunctional organic compound having at least three Functional groups as a branching agent in the preparation of a polycarbonate by the interfacial polycondensation process (i.e., the phosgene process), which comprises adding a solvent such as methylene chloride to an aqueous solution or a suspension of a sodium salt of a dihydric phenol and blowing phosgene thereinto. Although the branched polycarbonate thus prepared is improved in its moldability, not only does this process necessitate the use of highly toxic phosgene, but also chlorine remains in a reaction product mainly comprising a produced polycarbonate. When chlorine remains in the reaction product mainly comprising a polycarbonate, the reaction product mainly comprising a polycarbonate discolors during in the molding thereof at high temperatures. Accordingly, a step of washing the obtained reaction product is necessitated to lower the residual chlorine concentration.
As described above, there is known, besides the interfacial polycondensation process, the melt transesterification process which comprises heat-melting a dihydric phenol and a carbonic diester, such as diphenyl carbonate, and conducting the polycondensation through transesterification at a high temperature and under a reduced pressure, as a process for preparing a polycarbonate. The transesterification process has advantages in that the use of highly toxic phosgene is not necessitated and that it can dispense with the step of removing residual chlorine, and thus is a remarkably effective process for preparing a polycarbonate composition free from chlorine causative of discoloration.
However, the polycarbonate composition (or reaction product) prepared by the transesterification process is also contaminated with impurities causative of discoloration and the like. To overcome this problem, it is proposed in Japanese Patent Publication-A No. 4-100824 (published on Apr. 4, 1992) to use a diaryl carbonate having a xanthone content of 10 ppm or below. Although the use of such a carbonate can give a relatively light-colored polycarbonate, it has a problem in that the obtained polycarbonate is somewhat yellows as compared with that prepared by the phosgene process.
In the preparation of a polycarbonate by a transesterification process, the polycondensation is generally effected in the presence of a catalyst at high temperature under reduced pressure. As catalysts and which have a high activity and are useful for the preparation of a polycarbonate by the transesterification process, alkali metal compounds and alkaline earth metal compounds are well known. When a process using an alkali metal compound or an alkaline earth metal compound as a catalyst, among the transesterification processes for the preparation of a polycarbonate, is conducted at a high temperature, branched structures by the Kolbe-Schmitt reaction and/or through the formation of an isoalkenylphenol are formed [see L. Bottenbruch, Encyclopedia of Polymer Science and Technology. 10, 722 (1969)] as described above. However, the formation of branched structures by such side reactions are very difficult to control and the obtained polycarbonates exhibit extremely poor physical properties, because a structure different from that inherent in polycarbonate is incorporated into the polycarbonate by the formation of a branched structure. Further, the formation of branched structures by the above side reactions are closely connected with the discoloration of a polycarbonate (composition).
As described above, the formation of a high-molecular-weight polycarbonate which is free from discoloration by the transesterification process is generally difficult unlike by the interfacial polycondensation process.
To overcome the problems described above, many compounds have hitherto been proposed as transesterification catalysts. A process for the preparation of a polycarbonate using a catalyst system comprising a combination of a specific nitrogen-containing basic compound, i.e., quaternary ammonium hydroxide, with a borate is proposed, for example, in U.S. Pat. No. 4,590,257 (date of patent: May 20, 1986, assignee: General Electric), and a relatively light-colored polycarbonate composition can be obtained with the use of this catalyst. However, it has a problem that much time is necessary to give a high-molecular-weight polycarbonate since the activity of this catalyst is low.
Furthermore, it has also been known that a boron compound such as boric acid and triphenylborate is useful to prevent the discoloration of the polycarbonate [see U.S. Pat. Nos. 3442854 (assignee: FARBENFAB BAYER AG), 4590257 (assignee: GENERAL ELECTRIC CO.: date of patent: May 20, 1986) and 5276109 (assignee: GENERAL ELECTRIC CO.; date of patent: Jan. 4, 1994), and European Patent Publication-A Nos. 351168 (published on Jan. 17, 1990) and 435124 (published on Jul. 3, 1991)]. However, the use of the boron compound sometimes brings about other problem(s).
Accordingly, an object of the present invention is to provide a discoloration-Free and chlorine (which is causative of discoloration) Free (co)polycarbonate composition mainly comprising a high-molecular-weight, linear (co)polycarbonate which is free from any branched structure which might be formed by a side reaction and has excellent heat stability, and a process for the preparation thereof.
Another object of the present invention is to provide a (co)polycarbonate composition mainly comprising a heat-stable (co)polycarbonate having a branched structure formed not by a side reaction but by using a polyfunctional organic compound having at least three functional groups as a branching agent, which is useful for the production of a hollow molding, is not discolored and is free from chlorine causative of discoloration, and a process for the preparation thereof.